The present invention relates to a target device for marking inaccessible measurement points or measurement points which are difficult to sight with angle-measuring instruments.
Recently, methods of operating with geodetic angle-measuring instruments have been established for measuring larger industrial objects. Points to be measured on such objects are sighted by at least two different theodolites, and the coordinates of the target points are determined from the theodolite measurements.
For ease of recognition, it is known to characterize target points by adhesive markings, or by projecting a light spot onto the target point. This type of marking is, however, problematical when the sight line and the object surface to be measured form an acute angle, since in such case the target marks, which necessarily cover an area, are distorted in perspective and can no longer be brought into precise coincidence with the sighting cross hairs of the measuring instrument. This leads to inaccurate measurements.
It is furthermore known to use targets in the form of vertically suspended balls illuminated from below for measurement purposes in mining (West German Pat. No. 441,880). Such targets can, however, be used only for the marking of bottom sides, and their pendulum suspension renders them sensitive to vibration.